During many clinical procedures, a physician requires the reduction or complete stoppage of blood flow to a target region of the patient's body to achieve therapeutic benefit. A variety of devices are available to provide occlusion of blood vasculature including embolic coils, metal-mesh vascular plugs, beads, particles, and glues. Interventional radiologists and vascular surgeons (and similar medical specialists) draw from these choices based upon the specific need and confidence of a rapid and effective occlusion given the attributes and deficiencies of each of these options. These devices may be used to occlude vasculature in situations, for example, requiring treatment of arteriovenous malformations (AVMs), traumatic fistulae, some aneurysm repair, uterine fibroid, and tumor embolization. For these clinical treatments, the blood flow through a target section of a blood vessel must be stopped. The device is introduced into the blood vessel through a sterile delivery catheter or sheath using common percutaneous access outside the body. The delivered, artificial device, induces an initial reduction of blood flow through a simple mechanical blockage which in turn triggers the body's natural clotting process to form a more complete blockage comprised of the thrombus adhered to the device.
Current exemplary embolic coils are made from biocompatible materials, and provide a biodurable, stable blockage of blood flow. The coils anchor to the vessel wall through radial compliance pressing onto the vessel wall surface. Coils must be suitably anchored to avoid migrating downstream under the forces of the blood flow, which can be significant in larger vasculature. Embolic coils are often shaped for flexibility through the use of a primary coiling, and for achieving a “coil pack” within the vessel through the use of a secondary, sometimes complex, three dimensional shape. The coil pack appears as a relatively random crossing and intertwining of the coil within the vessel. After slowing the blood flow, over time, a clot forms around the embolic coil, and blood flow through the section is completely blocked.
Typical embolic coils are formed using two major steps: 1) a wire of platinum or other bio-compatible material is wound into a spring, forming what is commonly referred to as a primary coil; and 2) the primary coil is in turn wound around a mandrel having a more complex shape and then subjected to high heat (e.g., heat setting) to yield a secondary coil. The secondary coil thus is a coiled wire of complex-shape or, if helical, a larger curl diameter. Coils can also be provided in other secondary shapes, such as those having multiple helical curl diameters, and in tapered helical shapes with one end employing a large curl diameter and the other end a small curl diameter. These metal coils are straightened, within their elastic bending limit, so as to be advanced into a delivery catheter and pushed down the catheter by a guide wire, pusher, or a detachable pre-attached pusher, until expelled into the vessel. Often, polymeric fibers are applied to the metallic coils in order to increase a thrombus response in addition to providing a scaffolding for thrombi to adhere to and be retained on the coil.
Embolic coils are sized to fit within the inner lumen of a catheter or sheath to be delivered to the target occlusion site individually and sequentially. Typically, a physician will use multiple coils to occlude a single vessel and in some cases, especially for larger blood vessels (above 5 mm or so), the physician may use a significant number coils to achieve cessation of blood flow. To complete an occlusion procedure with embolic coils, the physician must sequentially reload the catheter with several individual coils until he/she has determined that the occlusion is sufficient. The physician typically determines whether sufficient coils have been deployed by assessing the level of occlusion of the vessel flow, e.g., by using contrast media in concert with typical medical imaging techniques. This “place and assess” method can extend the medical procedure time, expose the patient to increased levels of contrast agent, and increase radiation exposure to both the patient and the physician through extensive imaging.
Embolic coils are also known for challenges in achieving precise vascular placement. Many of these coils are simply pushed out of the end of a delivery catheter. The final coil pack location is dependent upon whether the coil has been properly sized prior to deployment or whether the coil was properly anchored into a side vessel/branch as prescribed by several of the coil manufacturers for greater confidence in the coil pack's final position. Both of these techniques require a high level of physician skill if there is a desire to accurately position both the distal and proximal faces of the coil pack in a vessel using sequential, pushable coils. Some of the coil manufacturers provide a detachable coil that, once properly placed, can be released from a delivery control wire at the user's discretion. If the coil is not in the preferred location, it can be retracted and replaced if needed to achieve better position. However, only the proximal end of the coil is attached to this control wire resulting in only indirect control of the position of the coil pack's distal face.
Using coils for embolization can present other unique challenges. Voids in the coil pack, developed either during the procedure or post operatively, can cause channels and resulting blood flow in an unintended area. This condition is typically referred to as recanalization. Depending upon the significance of the condition, e.g., internal hemorrhage, retreatment or surgical intervention may be necessary. The ability to quickly and reliably develop a consistently dense coil pack in a vessel is a key to a successful vascular occlusion product.
Also, embolic coils can be easily misplaced. Embolic coils may either be injected through a delivery catheter with a syringe filled with saline, pushed by an independent guide wire, or deployed with a detachable pusher that is only connected to the coil via its proximal end. The coil pack shape is dependent upon the successful placement of the initial coil. Therefore, coils can easily be misplaced, should the initial coil not land correctly or be slightly undersized to the target vessel and slip beyond the target location. As such, embolic coil packs are known for a high propensity of being elongated in overall size. While these devices have been employed clinically for years, coils reflect significant challenges when attempting to embolize in a very precise or limited section of vasculature.
Metal mesh vascular plug devices have also been developed and commercialized to achieve vascular occlusion. These devices achieve occlusion with a single deployment using a metal mesh to provide mechanical flow blockage and, after some time, a thrombus forms and a complete occlusion results. When deployed, these devices appear like metal mesh balloons or baskets, with one or more lobes contacting the vascular wall, but with defined proximal and distal faces. With occlusion occurring after a single device deployment, these products address many of the deficiencies of embolic coils. However, due to the porosity of the mesh basket and the lack of the polymeric fibers used in coils, the metal mesh plugs have been shown to take longer to achieve occlusion than a properly placed embolic coil pack.
Further, these metal mesh devices are relatively stiff due to their construction and have limited ability to traverse the sharp turns found in catheters that have been placed in a highly tortuous vascular path. The mesh is collapsed into a narrow tube-like shape for introduction and deployment through a delivery catheter or sheath before expanding into the balloon-like shape upon deployment. This narrow tube-like shape allows the device to be delivered in the central lumen of small catheters or sheaths similar to coils. However, when the mesh is collapsed, it elongates and becomes a fairly rigid tubular structure. So while being capable of entry into a small delivery catheter, it has a limited ability to traverse the sharp turns found in highly tortuous paths to the target vessel. Subsequently, the advantages of a single occlusion device are offset by the slow occlusion performance and limited application to occlusion target sites that have non-tortuous access.
The information included in this Background section, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the claims is to be bound.